(i) When the columns are very near to each other so that their footings overlap.
(ii) When the bearing capacity of the soil is less, requiring more area under individual footing.
(iii) When the end column is near a propertv line so that its footing cannol spread in that direction.
A combined footing may be rectangular or trapezoidal in plan. The aim is to get uniform pressure distribution under the footing. For this, the centre of gravity (CG.) of the footing area should coincide with the CG. of the combined loads of the two columns, If the outer column, near the property line, carries heavier load provisión of trapezoidal column becomes essential to bring the C.G. of footing in line with the C.G. of the two column loads. In other cases, a rectangular footing may be preferred.
1. Combined rectangular footing.
2. Combined trapezoidal footing.
3. Continuous Footing.